1. Field of the Invention
The subject invention is directed to a process for recovering polyester-grade ethylene glycol from crude ethylene glycol, such as the product mixture obtained by hydrogenating dialkyl oxalate.
2. Description of Related Art
The preparation of ethylene glycol is of particular interest to the chemical industry because of the varied uses of this compound. A particularly important use of ethylene glycol is in the production of polyester fibers. Ethylene glycol used in the manufacture of polyester fibers generally must be of exceptionally high purity because even a small quantity of impurity may have a deleterious effect on the resulting polyester fiber. Ethylene glycol also finds application in deicing fluids, antifreeze, hydraulic fluids, and in the manufacture of alkyd resins and solvents.
Ethylene glycol may be prepared by vapor phase catalytic hydrogenation of dialkyl oxalate. This reaction may be represented as shown below: ##STR1##
It is believed that hydrogenation of dialkyl oxalate actually proceeds step-wise according to the following two equations: EQU (II) (COOR).sub.2 +2H.sub.2 .fwdarw.HOCH.sub.2 COOR+ROH EQU (III) HOCH.sub.2 COOR+2H.sub.2 .fwdarw.HOCH.sub.2 CH.sub.2 OH+ROH
Reference frequently is made herein to the use of dimethyl oxalate (DMO) as the dialkyl oxalate reactant. Since hydrogenation of DMO has been studied in some detail, much of the following discussion describes the behavior of the hydrogenation reaction and subsequent treatment of the hydrogenation product in terms of the hydrogenation of DMO. The following discussion is not, however, intended to be limited to recovery of ethylene glycol formed by DMO hydrogenation.
The ability to increase the purity of ethylene glycol product is of particular interest because impurities, even in small amounts, may render the ethylene glycol unsuitable for manufacturing polyester fiber. When ethylene glycol contains even small quantities of impurities, the properties of the polyester produced, such as fiber dyeing characteristics, fiber strength, fiber color, etcetera, generally are affected adversely. High purity ethylene glycol suitable for use in the product of polyester fiber is referred to as polyester-grade ethylene glycol. Substantially pure ethylene glycol not suitable for use in the production of polyester-grade fiber generally is referred to as antifreeze-grade ethylene glycol.
The product mixture from the hydrogenation of dialkyl oxalate will contain unreacted dialkyl oxalate, partially hydrogenated dialkyl oxalate, i.e., glycolate (HOCH.sub.2 COOR), alcohol, ethylene glycol and other impurities. For example, in the case of DMO hydrogenation, the product mixture contains ethylene glycol, methanol, unreacted DMO, methyl glycolate (MGA), water, 1,2-butanediol (1,2-BD), various impurities having normal boiling points lower than that of ethylene glycol, and various other impurities having normal boiling points higher than that of ethylene glycol (referred to herein as heavies).
A relatively straightforward process of recovering ethylene glycol from a DMO hydrogenation product mixture employs a three-column purification system wherein the lowest-boiling material such as methanol is removed as overhead in a first fractionation column, while the column bottoms are treated in a second fractionation column where intermediate-boiling materials, such as MGA, DMO, water and 1,2-BD are removed as overhead. Finally, ethylene glycol is recovered by treating the bottoms of the second column in a third fractionation column. In the third column antifreeze-grade ethylene glycol can be recovered in the overhead, polyester-grade ethylene glycol can be recovered in a side stream, and a tails stream comprising heavies is withdrawn from the column bottom.
In order for a sample of ethylene glycol to qualify as polyester-grade ethylene glycol, the sample must pass a stringent ultraviolet light (UV) transmittance test. This test is conducted by comparing the transmittance of ultraviolet light at designated wavelengths through samples of ethylene glycol and of distilled water. The amount of ultraviolet light transmitted through a one centimeter thick ethylene glycol sample, divided by the amount of ultraviolet light transmitted through a sample of distilled water of similar thickness, converted to a percentage, constitutes the percent transmittance of an ethylene glycol sample. Current commercial fiber-grade ethylene glycol ultraviolet light transmittance specifications are listed below: